Make flags-sdk skill setup-first for discovery (#487)

* Make flags-sdk skill setup-first for discovery

Lead the skill description and structure with install/setup so agents load it for configuration, not only usage. Split setup vs create-flag workflows and sync CLI guidance to create/rules plus timestamp epoch-ms notes (EXP-3256).

* Widen skill title to cover setup and usage

* Drop timestamp CLI notes from this PR

* Keep create vs add CLI rename out of this PR

* Fix setup order and restore skill discovery triggers

Create flags.ts before Flags Explorer, pull FLAGS during configure, and restore frontmatter usage triggers dropped in the setup-first rewrite.

* Shrink skill description under 1024-char limit

Keep setup-first and restored triggers; trim wording so Skills CI validation passes.

* Harden flags-sdk skill setup guidance

Use export {} for empty flags.ts, note Explorer import path when flags are not at root, and clarify FLAGS_SECRET for setup-only.

* Remove Hypertune from flags-sdk skill

Hypertune is no longer supported; drop it from the skill description, references list, and providers guide.